fix: do not set empty token so anon works for public ecr

[natefaerber]

Fixes #167

The TokenAuth object has a None value for token until a login is executed. AWS ECR returns a 400 Bad Request when presented with Authorization: Bearer None while Docker Hub returns a 401 Unauthorized with proper www-authenticate header.

This patch simply skips adding Authorization: Bearer None which AWS ECR seems to expect. Docker Hub seems to handle the lack of Authorization header the same as if it got Bearer None.

My testing of this patch follows.

>>> from oras.logger import setup_logger, logger
>>> setup_logger(quiet=False, debug=True)
>>> from oras.client import OrasClient
>>> client = OrasClient('public.ecr.aws')
>>> client.get_tags('/aws-controllers-k8s/acm-chart')
Requesting anon token with params: {'service': 'public.ecr.aws', 'scope': 'aws'}
Successfully obtained anonymous token!

client = OrasClient('registry.hub.docker.com')
client.get_tags('openresty/openresty', N=10)
['0.0.13', '0.0.19', '0.0.20', 'v0.0.6', '0.0.4', '0.0.5', '0.0.11', '0.0.7', 'v0.0.4', '0.0.17', '0.0.16', 'v0.0.3', '0.0.6', 'v0.0.5', 'v0.0.1', '0.0.10', 'v0.0.2', '0.0.2', '0.0.8', '0.0.18', '0.0.3', '0.0.1', '0.0.15', '1.0.0', '0.0.12', '0.0.9', '0.0.14']
>>>
>>> client = OrasClient('registry.hub.docker.com')
>>> client.get_tags('openresty/openresty', N=10)
Requesting anon token with params: {'service': 'registry.docker.io', 'scope': 'repository:openresty/openresty:pull'}
Successfully obtained anonymous token!
['1.11.2.1-alpine', '1.11.2.1-centos', '1.11.2.1-centos-rpm', '1.11.2.1-trusty', '1.11.2.1-xenial', '1.11.2.2-alpine', '1.11.2.2-alpine-fat', '1.11.2.2-centos', '1.11.2.2-centos-rpm', '1.11.2.2-jessie']
>>>